Microcavity plasma devices produce a nonequilibrium, low temperature plasma within, and essentially confined to, a cavity having a characteristic dimension d below approximately 500 μm. This new class of plasma devices exhibits several properties that differ substantially from those of conventional, macroscopic plasma sources. Because of their small physical dimensions, microcavity plasmas normally operate at gas (or vapor) pressures considerably higher than those accessible to macroscopic devices. For example, plasma devices with a cylindrical microcavity having a diameter of 200-300 μm (or less) are capable of operation at rare gas (as well as N2 and other gases tested to date) pressures up to and beyond one atmosphere. In contrast, standard fluorescent lamps, for example, operate at pressures typically less than 1% of atmospheric pressure. High pressure operation of microcavity plasma devices is advantageous. It is well known, for example, that plasma chemistry at higher pressures favors the formation of several families of electronically-excited molecules, including the rare gas dimers (Xe2, Kr2, Ar2, . . . ) and the rare gas-halides (such as XeCl, ArF, and Kr2F) that are known to be efficient emitters of ultraviolet (UV), vacuum ultraviolet (VUV), and visible radiation. This characteristic, in combination with the ability of microplasma devices to operate with a wide range of gases or vapors (and combinations thereof), offers emission wavelengths extending over a broad spectral range. Furthermore, operation of the plasma in the vicinity of atmospheric pressure minimizes the pressure differential across the packaging material when a microplasma device or array is sealed.
Another unique feature of microplasma devices is the large power deposition into the plasma (typically tens of kW/cm3 or more), which is partially responsible for the efficient production of atoms and molecules that are well-known optical emitters. Consequently, because of the properties of microplasma devices, including the high pressure operation mentioned above and their electron and gas temperatures, microplasmas are efficient sources of optical radiation.
Microcavity plasma devices have been developed over the past decade for a wide variety of applications. An exemplary application for an array of microplasmas is in the area of displays. Since single cylindrical microplasma devices, for example, with a characteristic dimension (d) as small as 10 μm have been demonstrated, devices or groups of devices offer a spatial resolution that is desirable for a pixel in a display. In addition, the efficiency for generating, with a microcavity plasma device, the ultraviolet light at the heart of the plasma display panel (PDP) can exceed that of the discharge structure currently used in plasma televisions.
Early microplasma devices were driven by direct current (DC) voltages and exhibited short lifetimes for several reasons, including sputtering damage to the metal electrodes. Improvements in device design and fabrication have extended lifetimes significantly, but minimizing the cost of materials and the manufacture of large arrays continue to be key considerations. Also, more recently-developed microplasma devices excited by a time-varying voltage are preferable when lifetime is of primary concern.
Research by the present inventors and colleagues at the University of Illinois has pioneered and advanced the state of microcavity plasma devices. This work has resulted in practical devices with one or more important features and structures. Most of these devices are able to operate continuously with power loadings of tens of kW-cm−3 to beyond 100 kW-cm−3. One such device that has been realized is a multi-segment linear array of microplasmas designed for pumping optical amplifiers and lasers. Also, the ability to interface a gas (or vapor) phase plasma with the electron-hole plasma in a semiconductor has been demonstrated. Fabrication processes developed largely by the semiconductor and microelectromechanical systems (MEMs) communities have been adopted for fabricating many of the microcavity plasma devices. Use of silicon integrated circuit fabrication methods has further reduced the size and cost of microcavity plasma devices and arrays. Because of the batch nature of micromachining, not only are the performance characteristics of the devices improved, but the cost of fabricating large arrays is also reduced. The ability to fabricate large arrays with precise tolerances and high density makes these devices attractive for display applications.
This research by the present inventors and colleagues at the University of Illinois has resulted in exemplary practical devices. For example, semiconductor fabrication processes have been adopted to demonstrate densely packed arrays of microplasma devices exhibiting uniform emission characteristics. Arrays fabricated in silicon comprise as many as 250,000 microplasma devices in an active area of 25 cm2, each device in the array having an emitting aperture of typically 50 μm×50 μm. It has been demonstrated that such arrays can be used to excite phosphors in a manner analogous to plasma display panels, but with values of the luminous efficacy that are not presently achievable with conventional plasma display panels. Another important device is a microcavity plasma photodetector that exhibits high sensitivity. Phase locking of microplasmas dispersed in an array has also been demonstrated.
The following U.S. patents and patent applications describe microcavity plasma devices resulting from these research efforts. Published Applications: 20050148270-Microdischarge devices and arrays; 20040160162-Microdischarge devices and arrays; 20040100194-Microdischarge photodetectors; 20030132693-Microdischarge devices and arrays having tapered microcavities; U.S. Pat. No. 6,867,548-Microdischarge devices and arrays; U.S. Pat. No. 6,828,730-Microdischarge photodetectors; U.S. Pat. No. 6,815,891-Method and apparatus for exciting a microdischarge; U.S. Pat. No. 6,695,664-Microdischarge devices and arrays; U.S. Pat. No. 6,563,257-Multilayer ceramic microdischarge device; U.S. Pat. No. 6,541,915-High pressure arc lamp assisted start up device and method; U.S. Pat. No. 6,194,833-Microdischarge lamp and array; U.S. Pat. No. 6,139,384-Microdischarge lamp formation process; and U.S. Pat. No. 6,016,027-Microdischarge lamp.
Additional exemplary microcavity plasma devices are disclosed in U.S. Published Patent Application 2005/0269953, entitled “Phase Locked Microdischarge Array and AC, RF, or Pulse Excited Microdischarge”; U.S. Published Patent Application no. 2006/0038490, entitled “Microplasma Devices Excited by Interdigitated Electrodes;” U.S. Published Patent Application no. 2006/0071598, entitled “Microdischarge Devices with Encapsulated Electrodes,”; U.S. Published Patent Application no. 2006/0082319, entitled “Metal/Dielectric Multilayer Microdischarge Devices and Arrays”; and U.S. patent application Ser. No. 11/042,228, entitled “AC-Excited Microcavity Discharge Device and Method”, filed on Jan. 25, 2005.
The development of microcavity plasma devices continues, with an emphasis on the display market and the biomedical applications market. Widespread adoption of microcavity plasma devices in displays will hinge on several critical factors, including efficacy (discussed earlier), lifetime and addressability. Addressability, in particular, is vital in most display applications. For example, for a group of microcavity discharges to act as a pixel, each microplasma device must be individually addressable.